Method of making tin plate



3,0753% METHOD OF MAKING TIN PLATE Richard F. Higgs, Wilkinshurg, Pa.,assignor to United States Steel florporation, a corporation of NewJersey No Drawing. Filed May 27, 1960, Ser. No. 32,123 1 Claim. (Cl.204-42) This invention relates to a method of making tin plate and, inparticular, to the manufacture of electrolytic tin plate having improvedcorrosion resistance.

It is known that a controlled oxidation of cleaned, pickled sheet steel,prior to electrolytic tinning, will enhance the quality of the product.It is one of the objects of my invention to provide an improved methodof performing such oxidation. A further object is to provide a methodincluding such pre-oxidation of the steel followed by a lightelectrotinning thereof from an alkaline bath, then completion of thedesired deposit of tin from an acid bath. This latter procedure resultsin a product having markedly superior properties, particularly inrespect to corrosion resistance.

Generally speaking, my method comprises the formation of a film ofhydrated iron oxide on cleaned pickled steel sheet, immediately beforeelectrotinning it, by subjecting it to anodic electrolysis in a strongsolution of an alkali-metal hydroxide or carbonate. The film of oxide(Fe O -H O) should be from 100 to 500 A. thick so it will not becompletely removed by the acid electrolyte used for electrotinning thesteel base. The latter requires no other treatment except water rinsing,before electroplating, but I have found that a surprisingly improvedproduct is obtained by tinning the base first lightly from an alkalinebath and then depositing additional tin to the desired total amount froman acid electrolyte.

In a preferred practice of my invention, I take black plate, i.e.,low-carbon sheet steel, conveniently in the form of strip which has beencleaned and lightly pickled, and pass it through a water solution ofsodium hydroxide containing from 400 to 700 grams per liter of NaOH.Other alkali-metal hydroxides or alkali-metal carbonates may be usedinstead of NaOH. While the strip is in the solution, I make it anode ina circuit including a source of direct current and cause a current toflow therein at a density of from 100 to 300 amperes per square foot.The anodic treatment should continue until at least 200 coulombs persquare foot have been delivered. For the desired speed of travel of thestrip, the solution should be at a temperature of from 225 to 275 F.Within that range, the desired thickness of film, 100 to 500 A., isformed in from 1.5 to 4 seconds.

Further details of the invention the following tice thereof.

will be apparent from description of typical examples of the prac-Example 1 Black plate of tin-plate gage was cleaned of grease and dirtin an electrolytic alkaline cleaner of known type. The strip waswater-rinsed and then electrolytically pickled in a solution of 5%(weight) sulfuric acid at ambient temperature, using cathodic-anodicpolarity at a current density of 100 a.s.f. for 1.5 seconds at eachpolarity. The strip was then water-rinsed and treated electrolyticallyas anode, as shown by the following data:

Aqueous solution g./l NaOH 600 Temperature F 250 Current density a.s f100 Time seconds 4 I plated with tin from an acid bath as set 3,075,897Patented Jan. 29, 1963 Following the above treatment, the strip wasWater-rinsed and then tin plated in an aqueous solution consisting ofSn++ (as SnSO g./l 30 Phenosulfonic acid (calculated as sulfuric acid)Dihydroxy diphenyl sulfone g./l 6

under the following plating conditions:

Temperature F Current density q q f Solder Prune Iron solubond pack tionvalue strength liie (wks) (lb/in.)

Treated 27 48 26 Untreated 52 43 23 1 Lower values are better. 2 Highervalues are better.

Example 2 The same procedure as described in Example 1 was used exceptthat oxidation-treatment current density was increased to 184 a.s.f. Thefollowing results were obtained:

Iron Solder Prune solution bond pack value strength life (wks) (lb./in.)

Treated 21 51 27 Untreated 52 43 23 In other trials the improvement iniron solution value has averaged 50% and the improvement in solder bondstrength has averaged 15%. The available pack-life data are notextensive but indicate that a 25 to 30% improvement may be expected.

1 have found that the results obtained simply by oxidation of the blackplate before tinning, may be greatly exceeded by effecting the tincoating in two stages, viz., first applying a light coating by the knownalkaline electrotinning process and then plating the balance of thedesired total amount of tin by the known acid process.

In practicing this phase of my invention, black plate is cleaned,rinsed, pickled and again rinsed in accordance with conventionalelectrotinning practice. It is then subjected to oxidation of thesurface in any convenient manner, e.g., that described above, viz., in ahot, concentrated caustic solution wherein the steel strip is made theanodic pole. This oxidation step, as aforesaid, results in the formationof a hydrated iron oxide (Fe O -H O) on the surface of the strip havinga thickness of from 100 to 500 A. Following oxidation, the strip isrinsed and then plated with a flash coating of tin (0.05 lb./bb. orless) from a solution consisting of an alkali-metal stannate, eithersodium or potassium, and an alkali-metal hydroxide, such as sodium orpotassium. The strip is then rinsed and forth in Example 1, inconventional manner, to the desired final coating weight, usually 1.0lb./bb. tin.

The second stage of this phase of my invention, considered moreexplicitly, consists of depositing a thin coating of tin from analkaline tin-plating solution over the oxidized surface of the steelstrip without prior removal of the oxide film. For this purpose, Iprefer to use a conventional alkaline-tin electrolyte consisting ofabout 70 g./l. Sn++++ and 11 g./l. NaOH, at a temperature of 170 F. anda current density of about 30 a.s.f. The plating solution is not limitedto sodium stannate-sodium hydroxide, since a bath of potassiumstannate-potassium hydroxide or combinations of sodium and potassiumsalts could also be used. For my purpose, the amount of tin depositedfrom an alkaline bath need be no greater than 0.05 lb./bb. and can rangefrom about 0.005 to 0.05 lb./bb. I prefer that the coating be about 0.03lb./bb. alkaline tin.

Following the alkaline plating step by conventional tin plaing from anacid bath, the weight of the coating is increased to the desired finalvalue.

Typical examples of the practice outlined above are as follows:

Example 3 Aqueous solution g./l. Na'OH-.. 500 Temperature F... 255Current density a.s.f 200 Time seconds 2.5

Following the above treatment the strip was water rinsed and plated froman alkaline electrotinning bath to a coating weight of 0.05 lb./bb. tinunder the following conditions:

Aqueous solution E Temperature F 170 Current density a.s.f 30

The strip was then water rinsed and plated to a total in coating weightof 1.0 lb./bb. tin in an acid aqueous solution as follows:

Sn++ g./l 32 Phenolsulfonic acid (calculated as H 80 g./l. 18 Dihydroxydiphenyl sulfone g. l 5 Temperature F 100 The strip was then rinsed, andthe tin coating was refiowed by melting by conventional resistanceheating.

Since acid tin plate of improved corrosion resistance is ordinarilyneeded only for citrus products, a special test has been developed toindicate, without actually packing the food in cans, the corrosionperformance of tin plate in contact with citrus products. This test isthe 'alloy tin couple (ATC) test, which involves removing the free(unalloyed) tin from the specimen to an irontin alloy (FeSn layer,coupling the specimen with a piece of tin foil, immersing the specimenand foil in grapefruit juice for 20 hours, and measuring the currentbetween the alloy and the tin in microamperes per square centimeter ofexposed alloy. The current so obtained indicates the pack-lifeperformance that could be expected if the in plate were fabricated intocans, packed with citrus products, and stored until the cans failed. Inthis particular test, the lower the current, the greater the corrosionresistance of the tin plate.

Current densitya s f Untreated 4 The results of ATC tests made on tinplate treated as last described above and conventional tin plate(produced without the steps comprising my invention) are shown below:

Grapefruit-Juice Pack Lite Predicted from ATC Current, weeks AT C,la/cm;

Untreated Treated Example 4 Black plate was cleaned and pickled as inExample 3, except that the time for cleaning was reduced to 2 seconds.The oxidation treatment was carried out in a solution containing 600g./l. NaOH at 250 F. 3 seconds, at a current density of 100 a.s.f. Thealkaline tin coating was deposited under the conditions described inExample 3 except that the amount of tin deposited was only 0.005 lb./bb.This was followed by overplating in acid electrolyte (using a solutionas in Example 3) to a total coating weight of 1.0 lb./bb. tin. Theperformance of plate so produced as measured by the ATC testis shownbelow:

Grapefruit-Juice Pack Life Predicted from ATO Current, weeks ATO, an/cm.

Treated Thus even with a reduction in the intensity of the oxidationtreatment and a reduction in weight of the alkaline undercoating, theimprovement in predicted pack life was about This demonstrates that animprovement greater than the 70% ordinarily to be expected from usingthe combination of treatments can be achieved, even under conditionsless favorable than the optimum. It will be evident from the foregoingthat the invention provides a cheap yet effective method of forming anoxide film of predetermined thickness and composition which materiallyimproves the quality of the finished product made by electrolyticallytinning the oxide-filmed black plate. An even higher quality product,from the standpoint of corrosion resistance, is obtained when theoxide-filmed black plate is subjected first to a light tin coating froman alkaline bath andthe balance of the coating from an acid bath.

Although I have disclosed herein the preferred practice of my invention,I intend to cover as well any change or modification therein which maybe made without departing from the spirit and scope of the invention.

1 claim:

A method of making tin plate which comprises treating electrolyticallyas anode, sheet steel of tin-plate gage, in a water solution at from 225to 275 F. containing from 400 to 700 grams per liter of an alkali-metalcompound selected from the group consisting of hydroxides andcarbonates, at a current density of from to 300 amps.

5 per sq. ft. for from 1.5 to 4 seconds, thereby forming on the steel afilm of hydrated iron oxide from 100 to 500 A. thick, thenelectrodepositing over said film a first tin coating from an alkalinebath and then depositing over the first tin coating a second tin coatingfrom an acid 5 2,274,963

bath.

References Cited in the file of this patent UNITED STATES PATENTSOplinger Apr. 27, 1937 Hopper Mar. '3, 1942 Pink Nov. 24, 1942 UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,075,897January 29, 1963 Richard F. Higgs It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 3, line 20, for "plaing" read plating line 49, for "in", first,occurrence, read tin line 72,- i or "1n" read tin column 4, line 30,before "3 seconds". insert for Signed and sealed this 20th day of August1963,

(SEAL) Attest:

DAVID L. LADD Commissioner of Patents ERNEST W. SWIDER Attesting Officer

